Miners Toolbox Index Rock Mechanics  Discontinuities

Geologic Discontinuities

A discontinuity is a collective term referring to all structural breaks in rocks which usually have zero to low tensile strength.  Discontinuities can also be healed and infilled.  Discontinuities comprise joints, bedding, shears, contacts, veins, and faults.

Joints - A joint is a discontinuity which is relatively planar and on which there has been no displacement.  A series of joints in the same orientation are referred to as a joint set.  Joints may be open, healed, or filled. 

Bedding Joints - joints that are parallel to the bedding are referred to as bedding joints

Foliation Joints - joints that are parallel to metamorphic foliation

Shear - A structural break where differential movement has occurred.  The shear surfaces are characterized by the presence of slickensides, gouge, breccia, mylonite, or a combination of these.  Shear are in effect small faults and typically have displacements of less than 5 cm.

Fault - A shear with significant continuity and evidence of large displacement.  A fault can range from cm in width to a zone that is tens of metres thick.  The fault may contain breccia, gouge, crushed rock.  Fault zones are typically conduits for high groundwater flow.

Contact - a geologic contact between two distinct lithologic units

Vein - an infilling to a discontinuity caused by circulation of mineralized fluid and deposition of minerals.  Veins can cause healing of the original discontinuity 

Discontinuity Mapping Codes